In automation technology, field devices are often used, to serve for measuring and/or influencing process variables. Examples of such field devices are fill-level measuring devices, mass-flow measuring devices, pressure and temperature measuring devices, etc., which, as sensors, register the corresponding process variables, fill-level, flow (e.g. flow rate), pressure and temperature.
Field devices serving as actuators, to influence process variables, include, e.g., valves for controlling flow of a liquid in a section of pipeline or pumps for controlling fill level in a container.
A large number of such field devices are manufactured and sold by the firm, Endress+Hauser.
As a rule, field devices in modern manufacturing plants are connected via communication systems (HART, Profibus, Foundation Fieldbus, etc.) with superordinated units (e.g. control systems or control units). These superordinated units serve, among other things, for process control, process visualizing, process monitoring, as well as for tasks such as commissioning and other servicing of field devices.
Also falling under the heading “field devices” are, in general, such units (e.g. remote I/Os, gateways, linking devices) as are directly connected to a fieldbus and serve for communication with the superordinated units.
Fieldbus systems can be integrated into company networks. In this way, process and field device data can be accessed from different areas of an enterprise.
For worldwide communication, company networks can be connected also with public networks, e.g. the Internet.
The servicing of field devices requires corresponding operating programs. These operating programs can run independently in the superordinated units (e.g. FieldCare, Endress+Hauser; Pactware; AMS, Emerson; Simatic PDM, Siemens) or they can be integrated into control system applications (e.g. Simatic PCS7, Siemens; ABB Symphony; Delta V, Emerson). Besides control system applications, also operating programs, in part, already exhibit functionalities as regards plant monitoring (asset management).
A user places, in principle, two completely different requirements on fieldbus systems. One of these is process control, and the other is plant monitoring, or plant visualizing.
In the case of the fieldbus system PROFIBUS®, process control occurs via a cyclic data transfer. In such case, field devices exchange measured values at regular intervals with a process control unit (Controller), which generates corresponding control commands, e.g. for the pertinent actuators. As is known, in the case of PROFIBUS, communication between process control unit and field device is done using the Master-Slave-principle, wherein the process control unit functions as master and the field devices as slaves. Control of bus access is managed by the master, which sends control commands to the individual field devices via polling telegrams. The field device is permitted to transmit measured values to the master in a response telegram only after it has first received a polling telegram. The sequence of polling telegram and response telegram is executed by the master in time sequence with each of the slaves associated with it. The time until, in this way, each of the slaves associated with the master has been processed is referred to as the cycle time. Following the end of a cycle, the master then begins a new cycle. Process control systems are microprocessor-based, automation devices, whose control functionality is specified by automation software produced by automation personnel in the course of an engineering procedure and executed in the automation device in real time.
Programs or parts of programs having functionalities as regards plant monitoring are referred to in the following as asset-management-systems. These serve for processing device information for monitoring and optimizing device- and plant-states. The communication of data for asset-management purposes is done, as a rule, in the acyclic data traffic, for which a certain time interval is available between two succeeding cycles. The updating of data for purposes of asset-management happens, compared with the cyclic data traffic, in which an updating occurs after each cycle time, markedly slower.
Visualizing systems present the current parameters of the process on a user interface. Frequently, for the visualizing, besides the numerical values of a process variable and its units, also changed graphical symbols, such as dials, bars or trend charts, are used. Visualizing systems draw the data, most often, from process control systems. The visualizing system is, in the context of a user interface, also capable of influencing certain variables in the control system, which, in turn can itself have an influence on the course of the control program or the process.
Since process control tasks have precedence, the time interval available for the acyclic data traffic is markedly shorter than the time interval for the cyclic traffic.
As regards diagnostic data, one distinguishes between event-dependent, diagnostic data and diagnostic data read out only upon request e.g. of an asset-management-system. In the case of the fieldbus system, PROFIBUS, event-dependent, diagnostic data are bound to the communication partner cyclically communicating with the slave devices assigned to it (PROFIBUS Master Class 1).
This assures that a process control unit having cyclic data exchange with the slave devices reacts in near time to the diagnostic results. An asset-management-system connected with a control system application can be prompted by such application to fetch acyclic, diagnostic data. Therewith, the advantages of the event-dependent diagnosis (short reaction time) can be coupled with the desire for detailed information (additional, diagnostic data, which are communicated acyclically). While the diagnostic data communicated in place of the process data are of Boolean nature (to each bit corresponds a piece of diagnostic data, which can be identified by the device), the acyclically transmitted, diagnostically relevant data can include e.g. the degree of wear of a probe or the degree of fouling of the measuring device.
Proprietary solutions are known for the combination of process control and asset-management-system (for example, SIMATIC PCS7, of the firm, Siemens).
An asset-management-system independent of the process control can only request diagnostic data in the acyclic data traffic. Since, in this case, the asset-management-system is not informed of diagnostic events by a process control, it is necessary that diagnostic data be regularly requested, in order that diagnostic events can even be noticed by the asset-management-system.
Especially in the case of plants having a significant number of installed field devices and a multiplicity of diagnostically relevant parameters to be monitored, it is a fact that the updating of these parameters in the asset-management-system takes a very long time, so that long reaction times result. These long reaction times are very disadvantageous, especially in the case of critical diagnostic events. The long reaction times can even compromise the ability of the asset management systems to function.
The terminology “diagnostic event” is meant to include not only diagnostic reports from field devices but also issuance of alarms.